For over thirty years, poly(methyl methacrylate) ("PMMA"), has been used as a bone cement for the fixation of orthopaedic prosthesis during joint replacement surgery. However, several problems are associated with the use of PMMA bone cement. First, thermally-induced bone necrosis has been observed adjacent to the area containing the bone cement. This necrosis has been implicated as a possible cause of prosthesis loosening. See Lautheschlager, er al., Biomed. Mater. Symp. (1974) 5:185. Second, PMMA has a low modulus and shear strength when compared to metal alloys or even natural bone. See Black, J., Orthopaedic Biomaterials in Research and Practice, Churchill Livingstone, New York (1988). While the low modulus of PMMA bone cement was initially thought to provide a "cushioning" effect by absorbing the full brunt of compressive load by elastic deformation, the modulus mismatch between the prosthesis, bone cement, and bone is believed to cause stress shielding in adjacent bone which results in a mechanically-induced resorption of tissue and eventual implant loosening. Id. Further, deterioration of the metal/cement and cement/bone interfaces, which are completely mechanical in nature and lack chemical bonding between components, are believed to cause the loosening of the prosthetic device. Id.
PMMA also exhibits poor fatigue-crack resistance and fracture toughness and frequently fractures. See, Weber, F. A., et al., J. Bone Jt. Surg. (1975), 57B: 151; Charnley, J, Clin. Orthop. Rel. Res. (1975), 111:105; Weber, F. A. et al., J. Bone J. Surg., (1975):57B 151. The fracturing of PMMA bone cement may be caused by PMMA's low tensile and shear strength, by the porosity of the cement or non-uniform thickness of the cement or by the residual tensile stresses that are induced by shrinkage of the cement upon curing. See, Ahmed, A. M., et al., Biomed. Mater. Res. Symp. Trans. (1977) 1:56. When a prosthetic device loosens, because of bone resorption, interfacial degradation or cement fragmentation, it begins to move. The movement of the prosthetic device causes pain for the patient. The only way to relieve this pain is to replace the prosthetic device.
A myriad assortment of synthetic bone substitutes and artificial bone biomaterials are known. Many of these materials are employed in place of traditional bone grafts in those instances where missing or surgically-removed bone tissue must be replaced. For example, synthetic bone is used in the resection of bone tumors, replacement of bone fragmented due to trauma, bone non-union, or in the repair of congenital defects. Typically, bone substitutes fall into two categories: nonresorbable materials, which are meant to permanently replace defects and fully or partially resorbable materials, which are intended to provide an osteoinductive template for new bone regeneration.
Xenologous bone tissue can be calcined at temperatures in excess of several hundred degrees centigrade to remove proteinaceous components which contribute to immune rejection in conventional bone grafts (Abdelfattah, W. I., et al., Thermoch. Acta, (1993), 218:465; Rapanti, M. et al., Biomaterials, (1994) 15:433). This reconstituted bone, which is usually from bovine sources, may be used in the form of a powder as an osteoinductive template for hard-tissue augmentation in a manner similar to conventional bone grafts. High temperature calcining generally results in the transformation of the original microstructure into dense, tightly packed crystals of hydroxyapatite. However, treatment at temperatures below 500.degree. C. have been found to preserve the mineral architecture of the starting bone material while removing all of the organic components. See, Rapanti, M., et al., Biomaterials (1994) 15:433.
Current efforts have been directed to improving the rate of bone osseointegration in cementless systems by applying highly crystalline calcium phosphate coatings to the surface of porpous metal implants. The phosphate coatings are believed to invite bone growth but also consist of a brittle ceramic phase that cannot be fully resorbed. In addition, it is believed that the high processing temperatures needed to apply these coatings results in the degradation of the mechanical properties of the metal substrate. See, Smith, T., J. Min. Met. Mat. Soc., (1994) 46:54.
Alternative methods being employed involve the insertion of allografts or reconstituted bone at the metal/bone interface in an effort to invite bone regeneration. However, these methods exhibit many of the same problems associated with traditional bone graft procedures. Synthetic bone substitutes such as synthetically prepared calcium phosphates, resorbable polymer templates and conventional ceramic/polymer compositions, may not be sufficiently akin to natural tissue in the microstructure and/or composition to effectively improve growth rates.
Therefore, there is a need in the art to develop a method for growing fully or at least partially resorbable synthetic bone on the surface of a porous orthopaedic implant that can be used to stimulate cellular activity and bone growth that will eventually be replaced at least in part by natural bone tissue during osseointegration thereby eliminating the presence of a lingering mechanically brittle pure ceramic phase.